People spend a significant amount of time indoors, which has caused more concern about the quality of the air we breathe while inside buildings. Products and materials in homes and office buildings emit pollutants and chemicals, many of which are considered unhealthy or even carcinogenic. Recent studies by the U.S. Environmental Protection Agency and other health agencies have shown that indoor air pollutants are typically two to five times (sometimes 10 to 100 times) higher than levels found in outside air. One class of pollutants that has drawn particular attention is volatile organic compounds (VOCs). These compounds are ubiquitous, and much attention has been given to identifying the source of VOCs and reducing their emissions.
Generally, VOCs are volatile compounds that contain the element carbon, but in some applicable regulations, the definition of VOCs excludes one or more of methane, carbon monoxide, carbon dioxide, carbonic acid, metallic carbides and carbonates, ammonium carbonate, and exempt compounds, such as methylene chloride and 1,1,1-trichlorethane. One known source of VOCs is products manufactured of polyvinyl chloride (PVC). It is known that PVC and other halogenated polymers are subject to deterioration or degradation when exposed to heat and light. Two major types of PVC degradation are hydrogen chloride (HCl) degradation and oxidation. PVC generates HCl due to heat or short-waved light such as ultraviolet waves, X-ray, or γ-ray, resulting in double bonds within molecules. Oxygen in the air will cause oxidation reactions, resulting in chain scission or cross-linking. Such degradation results in generation of HCl, as well as such physical changes as darkening or other color change of the PVC polymer and the loss of tensile, flexural, and impact strengths. PVC degradation can occur during processing, as well as post-production (for example, once the PVC product has been installed in an office or home environment).
When PVC is processed at high temperatures, it is degraded by dehydrochlorination, chain scission, and crosslinking of macromolecules. Free HCl evolves and discoloration of the resin occurs along with important changes in physical and chemical properties. The evolution of HCl takes place by elimination from the polymer backbone; discoloration results from the formation of conjugated polyene sequences of 5 to 30 double bonds (primary reactions). Subsequent reactions of highly reactive conjugated polyenes crosslink or cleave the polymer chain, and form benzene and condensed and/or alkylated benzenes in trace amounts depending upon temperature and available oxygen (secondary reactions).
Stabilizer components have been used in PVC polymer compositions to reduce degradation of the polymer by neutralizing hydrochloric acid and accepting radicals generated by break down of the polymer chain. The chief purpose of a heat stabilizer is to prevent discoloration during processing of the resin compound. Degradation of the PVC polymer begins with the evolution of hydrogen chloride at about 200° F., increasing sharply with time and temperature. The most effective stabilizers have been found to be metal soaps, organo tin compounds, and epoxides.
Typically, stabilizers are provided in combination with a diluent. Typical diluents include short chain alcohols (having fewer than twelve carbon atoms, such as isodecyl alcohol), mineral spirits (this designation covers a variety of complex blends of differing solvating power, as known in the art), petroleum distillates, glycol ether, and the like. In some cases, the stabilizer is provided in a plasticizer as a diluent. Typical plasticizers include phthalates, epoxidized soybean oil, and other well-known plasticizers. The purpose of the diluent is generally to enhance solubility of the stabilizer in the PVC polymer and allow more rapid diffusion of the stabilizer in the polymer composition, as well as to enhance shelf stability of the stabilizer by reducing the incidence or amount of phase separation of the stabilizer component.
Despite inclusion of stabilizers and other additives to PVC, the polymer continues to emit VOCs post-processing, which in turn can lead to exposure to the VOCs for individuals who work or live in a building that contains PVC products. One the other hand, modification of the ingredients of the PVC to reduce emissions, for example in such PVC films as wallcoverings, can produce undesirable properties in the PVC product itself, such as reduced quality of the PVC film. As a result, despite efforts directed at VOC emissions, there is still room for improvement in reducing the amount of VOCs emitted from PVC products.
Efforts to detect VOC emissions from PVC products can focus on the identification and measurement of individual VOCs (IVOCs) and/or the total amount of VOCs (TVOCs) emitted from the product. Conventional PVC films have TVOC emission factors at 96 hours typically in the range of 4,500–5,000 μg/m2/hour, as measured by such methods as a Volatile Ingredient Evaluation (described herein). Typical VOCs emitted by PVC films include phenol, 2-ethylhexanoic acid, 3-tridecene, 3,7-dimethyl 3-octanol, 3,7-dimethyl 1-octanol, 1-dodecene, polychlorinated biphenyls, sulfur dioxide, ozone, unsaturated alcohols, benzene, toluene and xylenes, chlorinated hydrocarbons, acetaldehyde, C9 and C10 aliphatic alcohols, and n-tetradecane, n-pentadecane.
Despite the awareness of problems associated with VOCs, efforts to reduce TVOC emissions from PVC products to suitably low levels can be improved.